The present invention relates to an electronic musical keyboard instrument, and in particular to such an instrument having capture-type tone generators and means for phase locking the tone generators to avoid phase cancellation of octavely related tones.
Earlier electronic musical instruments, such as electronic organs, employed discrete keyers connected between the tone generator and the output circuitry and have a control input on which a keying envelope appears when the key corresponding to that keyer is depressed. Although discrete keyer arrangements permt a very large number of tones to be simultaneously played, they are quite costly due to the large number of keyers which must be provided. For example, in a typical sixty-one note manual having the usual number of footages, a total of ninety-six different keyers are necessary for each rank, and the ranks must be duplicated for various instruments such as brass and percussion.
With the advent of large-scale integration techniques, a large number of keyers can be incorporated into a single chip thereby reducing the cost of the keyers and facilitating their incorporation into existing organ circuitry. Keyers of this type still have the drawback, however, that a given keyer is dedicated to a certain tone thereby rendering the system somewhat inflexible, and since the keyers are an integral part of the semiconductor chip, changes cannot easily be made without a major redesign of the chip.
Since there are only a small number of keys, generally twelve or less, which can be played at any one time, the vast majority of the keyers in a discrete system are idle so that the system has a great deal of redundancy built into it. Many years ago, it was recognized that a single keyer could be controlled to produce a wide variety of tones, and if enough of these tone generators are provided, then normal polyphonic playing can be accomplished.
A problem which arises with capture-type tone generator systems is that the phase relationship between the same pitch in different octaves, or the same pitch in the same octave, cannot be controlled as it can in more conventional tone generation systems wherein the master oscillators themselves can be phase locked, or wherein the divider strings are driven by the same tone. Because a capture-type tone generator produces the pitch independently of the other tone generators, the notes played by two of the tone generators that may be of the same or octavely related frequencies can be sounded 180.degree. out of phase. This causes phase cancellation, which results in either a total loss of sound or a distorted sound, and is extremely undesirable from a musical standpoint. Even if the tones are not exactly 180.degree. out of phase, partial phase cancellation will occur unless the two tones are locked exactly in phase.
In one prior art capture tone generator organ, phase cancellation is compensated for by detecting when two of the master generators produce tones that are octavely related, and then controlling a slave generator to generate the same note as played by one of the master generators. This results in three independent tone generators producing the two octavely related notes, and since all three generators cannot be 180.degree. out of phase with each other, the possibility of total phase cancellation between octavely related notes is prevented. The drawback to this system is that a separate tone generator must be provided, thereby increasing the cost of the system. Moreover, it would seem that when two octavely related tones are played, the addition of the third tone generator producing one of them would result in a non-uniform sound as compared to the playing of two notes that are not octavely related. A further disadvantage is that the system does not compensate for partial phase cancellation, which can still occur to varying degrees even though a third tone generator is producing one of the notes.
A second type of phase locking system is used in synthesizers wherein two voltage controlled oscillators are activated by the depression of a single key on the keyboard. In this system, a synchronizing pulse is generated at the time that the key is depressed, and the tone generators are provided with a control input so that they start in the same phase in response to the synchronizing pulse.